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  #1  
Old 05-10-2011
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Dissimilar metals

Good Day.

[Hate to start a new thread, but can't seem to get below 35 PAGES of search results...a downfall of such a great forum.]

I want to put stainless steel eye strap on my boom (aluminum). Also want to put a cheek block on the aluminum mast. These will be hardware for a lazy jack system.

I'd rather use rivets than screws, but not sure if I should use stainless or aluminum rivets. I'd guess that stainless are stronger.

As to the title of this post, I'll be introducing dissimilar metals -- stainless and aluminum. So I know I need to use something like Tuf-Gel to help guard against galvanization. Correct so far?

My main question is, should I use aluminum rivets and put the Tuf-Gel between the base of the eye straps and spars, or use the stainless rivets and squeeze Tuf-Gel into the drilled holes (as well as between the eye strap base and spar).

Should I use Butyl tape between the stainless eye strap base and the spar? If I lay the tape over the drilled holes (hole to hole) that rivets will enter, I can then place the eye strap on the tape (which will separate the two dissimilar metals) and when I put the rivet in, won't it take a bit of the butyl tape into the drilled hole and thus protect the rivet from making contact with the aluminum spar?

Thanks.
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Old 05-10-2011
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Quote:
Originally Posted by NoWet View Post
Good Day.

[Hate to start a new thread, but can't seem to get below 35 PAGES of search results...a downfall of such a great forum.]

I want to put stainless steel eye strap on my boom (aluminum). Also want to put a cheek block on the aluminum mast. These will be hardware for a lazy jack system.

I'd rather use rivets than screws, but not sure if I should use stainless or aluminum rivets. I'd guess that stainless are stronger.

As to the title of this post, I'll be introducing dissimilar metals -- stainless and aluminum. So I know I need to use something like Tuf-Gel to help guard against galvanization. Correct so far?

My main question is, should I use aluminum rivets and put the Tuf-Gel between the base of the eye straps and spars, or use the stainless rivets and squeeze Tuf-Gel into the drilled holes (as well as between the eye strap base and spar).

Should I use Butyl tape between the stainless eye strap base and the spar? If I lay the tape over the drilled holes (hole to hole) that rivets will enter, I can then place the eye strap on the tape (which will separate the two dissimilar metals) and when I put the rivet in, won't it take a bit of the butyl tape into the drilled hole and thus protect the rivet from making contact with the aluminum spar?

Thanks.
I just installed a rigid boomvang, and used stainless rivets into the boom. I covered the end of the rivet with Lanocote before putting it in the hole, and also coated the bracket where it will be in contact with the boom.

If you look at the boom and the mast on my boat, there are many, many stainless fittings installed with stainless rivets. I assumed Bristol knew what they were doing and copied them with the boomvang.

I've no idea about the butyl tape - can anyone else chip in?
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Old 05-10-2011
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I wouldn't bother with the butyl tape, you are thinking too hard. A piece of electrical tape covering the base of the eyestrain or cheekblock. use Lanocoat or tef-gel for the fasteners. Remember we are not bedding them to the boom, just putting a layer between the two metals. Good luck!
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BTW, you are probably using 1/4" rivets and in stainless you need a serious riveter. I got one of these :

Amazon.com: Astro Pneumatic 1426 1/4-Inch Heavy-Duty Hand Riveter: Home Improvement

Made the job very easy.

Also be careful not to lose a mandrel inside the mast when you are starting the rivet. I did, and had to dremel the head off so I could push it through.
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Old 05-10-2011
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When using rivets there should not be any soft material between the stainless fitting and the aluminum spar unless it is a liguid like Tef-gel. the rivet can not be retightened at a later date and if you use a soft material between the two parts they will become loose with time and then need to be re-rivited. stainless rivets last much longer and are stronger then aluminum ones. aluminum rivets in an aluminum spar do not last and corrode away very fast compared to stainless
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Old 05-10-2011
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Good questions (and responses) - I am also considering rigid boom vang - so naive question - why not drill and tap, then use stainless bolts? The boom on my Pearson is fairly heavy/overbuilt.

Should the bolts be treated before use?

I noted the use of Lanocoat between the plate and the boom/mast...I wondered about that.
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My general rule of thumb is to use stainless rivets when attaching stainless fittings to aluminum. If I'm attaching plastic or aluminum fittings I will often use aluminum rivets. I coat the rivets with TufGel, Lanacote, Boatlife or even silicone and apply a coat between the fitting and the spar.
There is no problem drilling and tapping when the fasteners are subject to shear loads, but rivets are stronger if the fitting is being pulled away from the spar as they would be in flag halyards for example.
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Quote:
Originally Posted by paul323 View Post
Good questions (and responses) - I am also considering rigid boom vang - so naive question - why not drill and tap, then use stainless bolts? The boom on my Pearson is fairly heavy/overbuilt.

Should the bolts be treated before use?

I noted the use of Lanocoat between the plate and the boom/mast...I wondered about that.
I read all the threads I could find on the subject of threads vs. rivets.

When the metal is thin rivets would obviously have an advantage (too few threads in the metal), while in thick metal threads would be stronger. It looks to me like masts and booms are somewhere on the borderline - just enough threads for threading and bolting to work.

What I do know is that Bristol fastened the boom to the mast with S/S rivets, while small fittings like blocks are bolted to the boom.
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Old 05-11-2011
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The strongest rivets with the lowest electrolysis potential are Monel rivets. They're definitely way stronger than aluminium and I think they're stronger than stainless. Monel as a material is apparently subject to brinelling or work hardening but rivets are not a dynamic application so this is not a problem.

For high tensile applications I don't believe there is a substitute.

Read about it here if you're interested. Monel - Wikipedia, the free encyclopedia
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Originally Posted by Omatako View Post
The strongest rivets with the lowest electrolysis potential are Monel rivets. They're definitely way stronger than aluminium and I think they're stronger than stainless. Monel as a material is apparently subject to brinelling or work hardening but rivets are not a dynamic application so this is not a problem.

For high tensile applications I don't believe there is a substitute.

Read about it here if you're interested. Monel - Wikipedia, the free encyclopedia
monel by itself is very corrosion resistant but when used in aluminum is a bad thing. the aluminun will corrode in seawater about twice as fast as with stainless. monel is a nickel copper alloy with about 70% nickel and they make nickel aluminum batteries. monel is also not a strong as stainless and will work harden and become britle much sooner then stainless
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